Scientists Track Mosquitoes that Transmit Zika and Dengue by County

As mosquito-borne diseases that were once rare or unseen in the United States are making their presence known in the country, a team of researchers from the Centers for Disease Control and Prevention and Colorado State University is calling for greater efforts to systematically map the presence of the vectors carrying these illnesses.

Harvey Black

“The emerging threats of dengue, chikungunya, and Zika virus diseases have highlighted the need for accurate and up-to-date records for the geographical ranges of Ae. aegypti and Ae. albopictus to guide ongoing efforts to strengthen mosquito surveillance and control capacity and to serve as the basis for model-based predictions of future spread of these important arbovirus vectors,” the researchers wrote in an article in the Journal of Medical Entomology.

Senior author John-Paul Mutebi of the CDC and his colleagues reviewed data on the spread to the two mosquito species — Aedes aegypti and Aedes albopictus — over the past 21 years. The data are from a variety of sources, including CDC’s ArboNet, the scientific literature, and county collection records.

They show that these two species have expanded their ranges in the U.S. over the past two decades. They have gone from being found primarily in the Southwest and Southeast to the Mid-Atlantic states and as far north as New England.

This map shows counties where Aedes aegypti was reported between January 1, 1995 and March 2016. Counties shown in yellow had records for one year within that time period; those shown in orange had two years of presence records, and those shown in red had three or more years of presence records.

This map shows counties where Aedes albopictus was reported between January 1, 1995 and March 2016. Counties shown in yellow had records for one year within that time period; those shown in orange had two years of presence records, and those shown in red had three or more years of presence records.

But these data were not systematically collected, according to Mutebi, which led him to conclude that “we don’t know much about the distribution of Aedes aegypti and Aedes albopictus in the United States at this time.” Aedes aegypti transmits yellow fever, but when that disease ceased to be a problem, “surveillance progams moved away from those that are based on Aedes aegypti to those that are based on Culex, because now what we are worried about is West Nile [virus],” he said.

“The intensified surveillance for mosquitoes and mosquito-borne viruses that resulted from the 1999 introduction of West Nile virus to the United States therefore had very limited potential for generating data for the occurrence of Ae. aegypti and Ae. albopictus,” the authors wrote, because the traps that are suitable for Culex, don’t snare the two Aedes species.

That’s unfortunate because diseases like chikungunya and dengue are becoming increasingly prevalent in the United States. Chikungunya, which has no cure, can cause debilitating joint and muscle pain, fever, and rash. In 2014 a dozen locally transmitted cases were reported in Florida. The disease can cause worrisome problems not only for victims but for the those who care for them.

“It is one of those viruses that can end up in explosive epidemics that can overwhelm the health system,” Mutebi said.

“Conditions that facilitate sustained dengue transmission exist in the United States, and outbreaks have occurred during the past decade in Texas, Hawaii, and Florida. More outbreaks can also be expected in years to come,” wrote a team of researchers from Johns Hopkins University.

Options to control the spread of Aedes aegypti and Aedes albopictus include what Mutebi calls “environmental sanitation.” These mosquitoes lay their eggs in water in artificial containers — like tires, cans, and bottles — so it is important to remove these containers.

“It is labor intensive, it is inconvenient, but that is how you get rid of it,” he said.

Aedes aegypti, which has been present in the Unites States since the seventeenth century, is thoroughly “domesticated” and stays close to people. Making sure windows have screens is one way to keep them at bay.

Aedes albopictus, which first came to the United States in 1985, very likely in tire casings, is often found in forests, which makes it difficult to get rid of. It also breeds in artificial containers, making it imperative to get rid of such potential breeding sites.

Harvey Black is a freelance science writer. A long-time resident of Madison, Wisconsin, he has written for numerous publications including Environmental Health Perspectives, Scientific American Mind, New Scientist, The Scientist, and the Milwaukee Journal Sentinel.